Solid electrolyte ionic or mixed ionic-electronic conductors that can rapidly transport oxygen ions have a significant potential for use in air separation. Membranes made of such materials transport only oxygen ions and, therefore, have an infinite selectivity for the permeation of oxygen relative to all other species. This property is of particular advantage in the production of oxygen, since the product is inherently pure. Conversely, solid electrolyte materials may also be used to remove oxygen from an air stream to produce an oxygen-free "nitrogen" product.
Previously certain types of membranes, for example, organic polymer membranes, have been used to separate selected gases from air and other gas mixtures and this is an established technology. Composite hollow fibers which employ these organic polymer membranes may have separation factors that favor the permeation of oxygen over nitrogen by a factor of ten or less. Over the years, many processes employing such polymer membranes have been devised for the production of oxygen and particularly nitrogen from ambient air by taking advantage of this permeation differential. Examples of systems utilizing polymer membranes to separate oxygen from nitrogen are described in Prasad, U.S. Pat. No. 5,102,432, entitled Three-Stage Membrane Gas Separation Process and System; Prasad, U.S. Pat. No. 5,084,073, entitled Membrane Drying Process and System; and Prasad, U.S. Pat. No. 5,378,263, entitled High Purity Membrane Nitrogen.
Polymeric membranes are currently used for the commercial production of nitrogen from air. Typical membranes have O.sub.2 /N.sub.2 selectivities below 8 and can be used to make .about.95%-99.5% purity nitrogen efficiently in production apparatus having capacities of up to .about.30 tons per day. Although, in principle, polymeric membranes could be used to produce a high purity product by using more polymeric membrane stages, the required membrane area and power makes the process uneconomical. When a high purity nitrogen product is required, therefore, additional means must be employed to remove the residual oxygen in the retentate or product stream of the membrane process.
Air is a mixture of gases which may contain varying amounts of water vapor and, at sea level, has the following approximate composition by volume: oxygen (20.9%), nitrogen (78%), argon (0.94%), with the balance consisting of other trace gases. With use of a solid electrolyte process, oxygen is removed from the feed air stream but minor constituents or impurities in the feed air stream (for example, argon, carbon dioxide, water and trace hydrocarbons) are retained in the "nitrogen" product. Although most applications can tolerate the presence of argon, the other impurities such as moisture, carbon dioxide and hydrocarbons are generally undesirable in the product.
As discussed in more detail below, solid electrolyte membranes are most practical for removing small quantities of oxygen from a gas stream. Therefore, other processes in addition to the solid electrolyte process are usually required to produce a practical high purity nitrogen product. This invention provides efficient integrated processes incorporating solid electrolyte ionic conductors for the production of high purity nitrogen. At the present time solid electrolyte processes have yet to be used for the commercial production of nitrogen and the prior art is mute on combined processes for the removal of all the undesirable impurities in the production of nitrogen gas.
The combination of polymeric membrane purifiers or adsorption purifiers with a solid electrolyte membrane, however, can produce a high purity nitrogen product that is free of oxygen, moisture and hydrogen, in contrast with the nitrogen produced when the conventional deoxo process is employed for purification.
In addition, the use of a polymeric membrane or an adsorption purifier as a prepurifier in combination with a solid electrolyte membrane advantageously removes contaminants, such as carbon- and sulfur-containing compounds, which can poison or degrade the operation of the solid electrolyte membrane, before the gas stream containing such impurities reaches the solid electrolyte membrane. Such a prepurifier also has the additional advantage of removing water vapor, carbon dioxide and hydrocarbons that are generally undesirable contaminants in the final nitrogen product.
The adsorption process requires a purge stream for regenerating the adsorbent. In certain preferred embodiments of this invention, the waste stream from the ion transport process is used as the purge to the adsorbers. The operation of a polymeric membrane system for drying and impurity removal is also greatly enhanced when the permeate side is purged and the waste stream from the ion transport process is used as the purge gas.
Moreover, the use of the waste stream from the solid electrolyte membrane as a purge stream to reflux the adsorber or polymeric membrane purifiers enhances the efficiency of these operations in comparison with the separate stand-alone purification processes. Furthermore, since the solid electrolyte process operates at elevated temperatures, the heated waste stream can also supply heat to aid in the regeneration of prepurifier adsorbents. Thus, when the solid electrolyte process operates with a reactive purge, excess heat may be available for "thermal assisted" regeneration.
The polymeric membrane and adsorption systems are typically employed to prepurify the air stream that supplies the feed to the solid electrolyte membrane. Under certain circumstances, particularly when chemical reactions occur in the solid electrolyte membrane, it may be desirable to employ the polymeric membrane or adsorption system as a postpurifier for the nitrogen product stream. These postpurifiers can be operated in the same ways as the prepurifiers.
Membrane systems have long been used for the separation of nitrogen from air. Such membrane systems include the NitroGEN.TM. systems developed by Praxair, Inc. are for the commercial production of nitrogen from air. The purity of the nitrogen product depends on the number of permeation stages employed. For low purities, a single stage process suffices. Higher purity can be achieved in a two stage process wherein the permeate from the second stage (which is nitrogen-rich compared to air) is recycled to the feed compressor. By adding a third stage, with feedback of the permeate streams from the second and third stages, a still higher purity can be achieved. The oxygen content in the product nitrogen can be reduced to approximately 0.5% by these means, but the required membrane area and the system power both become excessive when higher purities are specified.
Usually when an oxygen-free product is specified, it is typical to add a conventional deoxo system to treat the retentate (product) from the membrane process. In the conventional deoxo process, a quantity of pure hydrogen is added to the retentate stream which then passes through a catalyst that induces the hydrogen to react with the contained oxygen to produce water. A separate drying system is required to remove this water. It is obvious that an excess of hydrogen (H.sub.2 &gt;2O.sub.2) is required. Among the other problems with the conventional deoxo method, an excess of hydrogen remains in the product nitrogen. The combination of a polymeric membrane system with a conventional deoxo postpurifying system represents the current state of the art for producing high purity nitrogen in small to medium quantities.
This invention combines known polymeric membrane processes or adsorption processes synergistically with the solid electrolyte process to produce a nitrogen product that is substantially free of all undesirable impurities. This invention, therefore, does not possess the disadvantages of the conventional deoxo process and improves the overall nitrogen purification process by combining solid electrolyte technology with the known advantages and simplicity of polymer membrane processes and adsorption processes for the production of high purity nitrogen.
Solid electrolyte membranes are made from inorganic oxides, typified by calcium- or yttrium-stabilized zirconium and analogous oxides having a fluorite or perovskite structure. As mentioned above, these oxide ceramic membranes are thus very attractive for use in new air separation processes. Although the potential for these oxide ceramic materials as gas separation membranes is great, there are certain problems in their use. The most obvious difficulty is that all of the known oxide ceramic materials exhibit appreciable oxygen ion conductivity only at elevated temperatures. They usually must be operated well above 500.degree. C., generally in the 600.degree. C.-900.degree. C. range. This limitation remains despite much research to find materials that work well at lower temperatures.
There are two types of ion transport membranes in use: ionic conductors that conduct only ions through the membrane and mixed conductors that conduct both ions and electrons through the membrane. As used herein, the terms "solid electrolyte ionic conductor", "solid electrolyte", "ionic conductor", and "ion transport membrane" are used to designate either an ionic-type system or a mixed conductor-type system unless otherwise specified. In the absence of a purge stream, the permeate stream that carries the oxygen away from the ion transport membrane is pure oxygen. For pressure-driven systems, both the feed and the retentate streams must be at a high pressure or the permeate stream must be at a very low pressure to create a driving force for the oxygen transport. For electrically-driven systems, a sufficiently high voltage may be applied to overcome unfavorable partial pressure systems although a purge will reduce the electrical power requirements considerably. While an unpurged membrane is attractive for the removal of larger quantities of oxygen from inert gas streams, the oxygen recovery is limited by pressures or electrical energy that can be applied. When a purge is used, the degree of purification that can be obtained is limited. Solid electrolyte technology is described in more detail in Prasad et al., U.S. Pat. No. 5,547,494, entitled Staged Electrolyte Membrane, which is hereby incorporated by reference to more fully describe the state of the art.
Advances in the state of the art of air separation using inorganic oxide membranes have been presented in the technical literature.
Patterson et al., U.S. Pat. No. 3,925,041, entitled Thermal Swing Gas Adsorber, describes a gas adsorbent vessel for use in a thermal swing prepurifier for air prior to cryogenic air separation.
Prasad, U.S. Pat. No. 4,931,070, entitled Process and System for the Production of Dry, High Purity Nitrogen, describes an air separation membrane process wherein membrane dryers are used to prepurify the air feed and/or postpurify the nitrogen product.
Haas et al., U.S. Pat. No. 5,004,482, entitled Production of Dry, High Purity Nitrogen, describes a membrane dryer used as a prepurifier in the air stream to PSA nitrogen production, or as a postpurifier in the nitrogen product stream from such a PSA system.
Prasad et al., U.S. Pat. No. 5,116,396, entitled Hybrid Prepurifier for Cryogenic Air Separation Plants, is a continuation-in-part of the application issued as U.S. Pat. No. 4,934,148 in which a membrane dryer is used in series with a PSA prepurifier for a cryogenic air separation system.
Jain et al., U.S. Pat. No. 5,156,657, entitled Process for Pre-Purification of Air for Separation, shows an example of a PSA system for prepurification of the air to an air separation unit.
Mazanec et al., U.S. Pat. No. 5,160,713 entitled Process for Separating Oxygen from an Oxygen-Containing Gas by Using a Bi-Containing Mixed Metal Oxide Membrane, relates to an oxygen separation process employing a bismuth-containing mixed metal oxide membrane which generally provides that the separated oxygen can be collected for recovery or reacted with an oxygen-consuming substance. The oxygen-depleted retentate is apparently discarded.
Mazanec et al., U.S. Pat. No. 5,306,411, entitled Solid Multi-Component Membranes, Electrochemical Reactor Components, Electrochemical Reactors and Use of Membranes, Reactor Components, and Reactor for Oxidation Reactions, relates to a number of uses of a solid electrolyte membrane in an electrochemical reactor. It is mentioned that nitrous oxides and sulfur oxides in flue or exhaust gases can be converted into nitrogen gas and elemental sulfur, respectively, and that a reactant gas such as light hydrocarbon gas can be mixed with an inert diluent gas which does not interfere with the desired reaction, although the reason for providing such a mixture is not stated. Neither of the Mazanec et al. patents cited disclose processes to produce a high purity product from an oxygen-containing stream.